1. Field of the Invention
This invention relates in general to charge transfer devices such as a CCD (charge coupled device) and in particular to an improved electrical charge transfer device in which large number of bits can be transferred in the CCD and in which the direction of charge transfer can be changed intermediate the input and output of the device.
2. Description of the Prior Art
CCD delay lines sometime are required with a large number of storage and transfer positions and if such devices capable of storing a large number of bits are arranged in a straight line, the semiconductor chip will become very large and it is difficult to design and manufacture it. For this reason, it is desirable that the channel be folded or bent intermediate its ends so as to change the charge transfer direction and therefore make the device in a more compact package.
FIGS. 1 and 2 illustrate prior art charge coupled devices in which the change of direction of charges intermediate the input and output occur. For example, in FIG. 1, a semi-conductor substrate 1 having a first conductivity type such as P type conductivity is provided with a channel stopper region 2 which consists of a higher impurity concentrated region of the first conductivity type and is formed on the surface of the substrate 1. Charge transfer portions 3 consists of a plurality of transfer regions 4 (4.phi.1, 4.phi.2) which are aligned in the charge transfer direction. In the embodiment of FIG. 1, the charges pass from the input 3A as shown by the arrow to the right then pass upwardly as shown by the curved arrow through the direction changing portion 5 to the output area 3B as shown by the arrow at the top of the Figure wherein the charges are moving to the left relative to FIG. 1. In the embodiment illustrated in FIG. 1, a 180.degree. of charge transfer direction occurs in the direction changing portion 5. The direction changing portion 5 is formed of a diffusion region 6 of a second conductivity type such as an N-type conductivity. Each of the transfer regions 4 is formed of a transfer electrode 8 such as 8.phi.1, 8.phi.2 which are formed on the substrate 1 over an insulating layer 7 as for example, of silicon dioxide. The electrodes 8.phi.1, 8.phi.2 are alternatingly arranged and driving clock pulses of .phi.1 and .phi.2 are applied to the alternate electrodes to provide charge transfer. An electrode 9 illustrated in FIG. 2a is provided on the substrate 1 over the insulating layer 7 in the region adjacent the direction changing portion 5 and a fixed DC voltage is applied to electrode 9 so as to fix the voltage of the diffusion region 6 between the regions 4.phi.1 and 4.phi.2 adjacent the diffusion region 6 of the direction changing portion 5. An electrical charge 12 will be sequentially transferred by the clock pulses .phi.1 and .phi.2 of the two phase driving signal through the transfer region 4 in a first direction, for example, to the right of the Figure then through the direction changing portion 5 and back in the opposite direction as illustrated by the arrow at the top of the Figure in FIG. 1. This transfer is accomplished by the potential 10 illustrated in FIG. 2a and potential 11 illustrated in FIG. 2b wherein it is noted that the charge 12 moves from the left of FIG. 2a to the right in FIG. 2b.
In the charge transfer devices such as described and illustrated in FIGS. 1 and 2, since the direction changing portion 5 is formed of the diffusion region 6, defects will be caused because the area occupied by the direction changing portion is relatively large and has a substantial length and the electrode 9 which receives the DC voltage is required so as to fix the voltage of the diffusion region 6 and thus the direction changing portion 5 becomes complicated in construction and it also becomes complicated to apply the drive voltage. In addition, since the electrodes 8.phi.1 and 8.phi.2 which overlie the transfer portions in both the areas 3A and 3B are connected to each other and receive the same driving pulses, however, they are offset from each other by one bit shift as illustrated by the diagonal portions of the electrodes extending between the regions 3A and 3B. Thus, the electrode 8.phi.1 in the region 3A is connected by an oblique portion 100 to the corresponding electrode 8.phi.1 in the region 3B. In order to prevent the portion 100 from being extremely narrow, the channel stopper 2 is made relatively long as indicated by the dimensions l1. Thus, the formation of the electrodes 8 is relatively complicated and the width l2 of the diffusion region 6 becomes wide which deteriorates the integrating density and adversely effects the loss and efficiency of charge transfer in the direction changing portion 5.
It has also been proposed to form the direction changing portion with a CCD. In such arrangements, the respective transfer electrodes in the direction changing portion are formed as sector shapes so as to form a channel path of generally similar semicircular configuration and hence to change the transfer direction of charges. This structure results in an increase in the number of bits of the CCD in the direction changing portion which increases and the result is that the charge transfer in the direction changing portion is apt to be disturbed and the direction changing portion occupies a greater area which lowers the integration density of the device.